1. Field of the Invention
The present invention relates to a method for calculating a probe mounting position in an on-machine measuring device attached to a machine tool as shape measurement of a workpiece is performed using the on-machine measuring device.
2. Description of the Related Art
Corrective machining based on on-machine measurement is essential for the achievement of shape accuracy on the order of nanometers in ultra-precision measurement. Recently, machined shapes that require corrective machining based on on-machine measurement have gradually become complicated. Therefore, shape accuracy on the order of nanometers also needs to be accurately achieved for shapes with sharp gradients of 60 degrees or more. Since the demand for simultaneous five-axis machining is expected to increase, moreover, it is apprehended that on-machine measurement of three-dimensional shapes based on simultaneous five-axis machining will have to be performed in the future.
A system configured to measure the shapes of measurement objects using an on-machine measuring device is disclosed in Japanese Patent Application Laid-Open No. 2010-32373. This system is characterized in that on-machine measurement of three-dimensional shapes based on simultaneous five-axis machining, as well as of shapes with sharp gradients of 60 degrees or more, can be achieved by performing simultaneous five-axis machining measurement with the additional use of a rotary axis in the conventional on-machine measurement based on linear axes only.
According to a prior art technique, a probe of an on-machine measuring device is mounted on a rotary axis in such a manner that its tip end portion is maximally aligned with the rotation center of the rotary axis on a plane perpendicular to the rotary axis, and adjusted by means of a size measuring device, such as a pair of calipers or a micrometer. Thereafter, a reference object (e.g., sphere) is measured and finely adjusted by using a stage, feed screw, hammer, etc., so that the probe tip end portion approaches the rotation center of the rotary axis. Similar adjustment or fine adjustment is also performed in the case where the probe is mounted off the rotation center.
In on-machine measurement with the additional use of a rotary axis, it is most important to accurately calculate the positional relationship between the position of a tip end of a probe and the rotation center of the rotary axis on which an on-machine measuring device is mounted. A method for calculating a probe mounting position using a flat plate is applied for a patent in Japan (Japanese Patent Application No. 2009-295501, filed Dec. 25, 2009).
If the relative probe mounting position is erroneous, the probe cannot be located during rotation measurement, so that accurate measurement on the order of nanometers cannot be performed.
Conventionally, however, manual adjustment needs to be repeated to locate the accurate probe mounting position. Accordingly, the adjustment takes much time and labor and depends considerably on the skill of an operator who performs the adjustment.
Further, the method for calculating a probe mounting position in an on-machine measuring device is based on arithmetic expressions, so that noise, error, etc., may be amplified during measurement. Thus, the calculated accuracy may be reduced, so that it needs to be corrected to ensure ultra-precision measurement that requires high accuracy.